AA alkaline batteries are widely used as main power sources for everyday items, toys, tools for hobbies, video game machines, potable mobile electronic equipment, and the like and are therefore desired to have long lifetime in using these tools.
It is necessary for elongating the driving period of tools to increase the discharge capacities of the batteries, which requires an increase in content of manganese dioxide as a positive electrode active material in the positive electrode. For increasing the content of manganese dioxide relative to a given volume, the content of graphite added as a conductive material (approximately 10% content in usual) must be reduced relatively.
However, graphite has a function as a binder for increasing the moldability of the positive electrode mixture in addition to a function of increasing the conductivity. Accordingly, reduction in content of graphite leads to loss of the conductivity and the moldability of the positive electrode.
For tackling this problem, various techniques for increasing the discharge capacity by increasing the content of manganese dioxide with the conductivity and the moldability of the positive electrode maintained are known, such as a technique in which the conventionally-used graphite is replaced by expanded graphite (see Japanese Unexamined Patent Application Publication 9-35719 and Japanese Unexamined Patent Application Publication 11-149927, for example), a technique in which the water content or the molding compressed density of the positive electrode mixture is optimized (see Japanese Unexamined Patent Application Publication 2000-306575, for example), and the like.
On the other hand, other countermeasures are provided in view of the fact that there is limit in increasing the content of manganese dioxide relative to a given volume. Namely, the discharge capacity is increased by increasing the outer diameter of the battery within the range provided in JIS to increase the volume of the battery while reducing the thickness of the battery case (see Japanese Unexamined Patent Application Translation of PCT Application) No. 2002-532851, for example).
For increasing the discharge capacity of an alkaline battery, it is effective to increase the internal volume by reducing the thickness of the battery case. Reduction in thickness of the battery case, however, lowers the crimping strength at the sealing part of the battery case to invite leakage of the electrolyte from the sealing part. Against this problem, a countermeasure has been taken in which the thickness of the opening part of the battery case is made thicker than that of the body part thereof (see Japanese Unexamined Patent Application Publication 2002-151017 and Japanese Unexamined Patent Application Publication 5-089861, for example).